In the manufacture of programmable bipolar circuits, such as PROM's, PLA's, PML's, and other PLD's, metallic fusible links are used as the programming elements. Historically, such fuses are fabricated from a sequence of metal depositions, maskings and etches followed by an interconnect deposition and photolithography. Recently, such fuses have been combined with barrier metal at the surface of semiconductor devices to form part of the interconnect. The sequence of formation is deposition of the barrier metal, deposition of the conductive metal, masking for the interconnect pattern, etching the conductive metal, stripping the resist, then remasking for the fuse pattern and etching the exposed barrier metal.
Also, processes have been devised using a somewhat different method for fuse fabrication in order to allow plasma etching of conductive layers, such as aluminum, for tighter interconnect pitches since dry etching of aluminum would etch into any fuse materials being formed of the same material as the barrier metal. In this sequence, after the barrier metal and conductive metal are deposited, a combined interconnect and fuse mask is used and the conductive layer and barrier layer are sequentially etched. Another mask that has an oversized hole is then applied and the conductive metal is wet etched to remove it from the fuse material.
For all of these techniques, fuse resistance is a complicated function of sheet resistance and fuse dimensions or width and length. Sheet resistance, which is resistivity divided by thickness, and fuse length are usually well controlled. However, fuse width is usually very narrow, especially for metals of TiW in order to meet circuit requirements for fuse resistance. This is because the TiW must also be sufficiently thick to serve as the barrier metal. Thus a critical dimension control is needed, but this pushes photolithography to the limits. Exposure and development is very critical as is the etch process, both for conductive metal, such as aluminum, as well as for fuse material of TiW.